1. Field of the Invention
This invention relates to a process for producing an expanded article of a crystalline thermoplastic resin. More particularly, this invention relates to a process for producing an expanded article of (1) a polypropylene resin, a polypropylene copolymer resin or a mixture of a polypropylene resin and less than about 50% by weight of an additional thermoplastic resin, or (2) a polyamide resin, which has excellent chemical and heat resistance and toughness.
2. Description of the Prior Art
Processes for producing a thermoplastic resin foamed article for use as a synthetic wood with woodgrain pattern by extruding a molten foamable thermoplastic resin through a nozzle having a number of apertures to expand the resin have already been known as described in, for example, U.S. Pat. Nos. 3,720,572 and 3,993,721 and Japanese Patent Application (OPI) No. 59,969/76.
The processes for producing a thermoplastic resin foamed article are directed to the production of a foamed resin primarily comprising a polystyrene resin. Since a polystyrene resin has a good foamability, desired resin foamed articles can be produced continuously over a long period of time according to the above-described processes.
However, in applying the above conventional processes to crystalline thermoplastic resins other than polystyrene, e.g., polypropylene, a resin containing polypropylene as a major component or a polyamide, it is very difficult to continuously produce resin foamed articles having good quality. No process suitable for foaming such crystalline thermoplastic resins have thus far been practically available. That is, since polypropylene and polyamide resins are crystalline thermoplastic resins, their melt viscosity is sensitively temperature-dependent, and a viscosity suitable for their expansion is in a narrow temperature range close to a temperature at which the resins crystallize. Generally, it is extremely difficult to conduct extrusion and expansion while controlling the nozzle temperature within such a narrow temperature range. In extruding such resins using a nozzle having a number of apertures, the resin streams flow in some portions with difficulty and partially crystallize and solidify to prevent expansion. In addition, such adverse affects on uniform, stable extrusion in respective apertures make it difficult to produce a resin foamed article having high quality using crystalline thermoplastic resin.
It has been considered that such problems could be solved by controlling the temperature of the nozzle with a high degree of accuracy in order to avoid crystallization and solidification of the resin. In fact, however, it is technically difficult to control the nozzle temperature within a critical narrow range suitable for expansion in order to avoid crystallization, since extrusion molding involves many factors which may vary widely. For example, to equip a nozzle with a temperature-controlling mechanism as disclosed in Japanese Patent Application (OPI) No. 59,969/76 is not satisfactory for a polypropylene resin or a polyamide resin; the resin crystallizes in part of a number of apertures, which prevents stable extrusion of resin streams. In addition, when the resin is cooled through a frame adjacent to a nozzle having a projection in its center as described in U.S. Pat. No. 3,993,721, it is difficult to extrude polypropylene resin quantitatively in respective apertures of the nozzle because plugging partly occurs. Thus, stable production of a thermoplastic resin foamed article, such as a polypropylene resin foamed article or a mixture thereof or a polyamide resin foamed article, having good quality over a long time was found to be very difficult.